The proposed project will use a combination of ethological (naturalistic observations) and experimental techniques to study the role of prelinguistic vocalizations in social interaction in 60 mother-infant pairs. The proposed research will bridge a gap between vocal development and socio-communicative development by 1) utilizing new methods for acoustic classification of infant prelinguistic vocalizations; 2) determining how phonologically distinct vocalizations influence maternal responsiveness, thereby identifying how they function in mother-infant interaction; and 3) exploring the influence of the nature and timing of maternal responses on infant vocal production. Given the dyadic nature of early communicative development, the research will be valuable in a clinical setting. Mothers suffering from depression and developmentally delayed children often do not react, as do unaffected mothers and infants. Without timely feedback, communicative interaction and socio-communicative development can suffer. Therefore, identifying the role of prelinguistic vocalizations in early mother-infant interaction can lead to the development of strategies for successful interaction in dyads in which one partner does not respond with expected or typical patterns.